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Re: Possible race condition or NULL pointer triggered by OQGraph


Hi Sergei

I cant, but Heinz has managed to (see

The value of lex at that point is NULL in the core

I guess if that situation is impossible by design, this is more likely to be a memory overrun instead...

In any case, I got Heinz to try my fix and the crash still happened in the same spot. So I think my next step at this point is valgrind...


On 02/06/14 20:53, Sergei Golubchik wrote:
> Hi, Andrew!
> On Jun 02, Andrew McDonnell wrote:
>> Hi all
>> (Cross-posted to oqgraph-developers, maria-developers)
>> I am trying to track down a segfault apparently triggered by concurrent
>> execution of queries through OQGraph - for context, see:
>> https://mariadb.atlassian.net/browse/MDEV-6282
>> I am however a bit confused as to what is going on, because at least some of
>> the information leads me to suspect that there could be more going on here
>> than just a simple race condition.
>> --Immediate Cause--
>> On the surface, a segfault appears to be caused in a method in sql_class.h,
>> Statement::check_limit_rows_examined() dereferencing a NULL pointer, `lex`.
>> The method check_limit_rows_examined() was called from sql_class.h method
>> handler::increment_statistics() .
> I don't see how thd->lex could be NULL there. It shouldn't be changed
> concurrently on anything.
> Can you actually repeat the crash?
> Regards,
> Sergei